Fronius modbus support

[MaStr]

Because I see the current Fronius web/time-of-use path and a direct Modbus path as two quite different backends.

The reverse-engineered web UI API feels a bit like hot lava to me, so I’d prefer an official Modbus-based path. Fronius documents Modbus here: https://manuals.fronius.com/html/4204102649/en-US.html

Happy to discuss it first though. Right now I’m mostly building tests and small internal pieces.

Originally posted by @filiplajszczak in #311 (comment)

[MaStr]

Yes, it is hot lava. 100% true.
I am glad, that it is working very stable now and it seems we worked around a lot of authentication issues.

I propose to implement it as an invertert type fronius-modbus.
And as Fronius seems to implement the sunspec modbus variant, maybe we are able to support other inverters via that variant

[filiplajszczak]

I’d keep the implementation Fronius-first for now, and only generalize toward broader SunSpec support once the behavior is proven.